The following hypotheses relating to the postnatal development of ammoniagenesis in rats will be tested. Glutamine may fail to provide ammonia because permeability barriers in the neonate prevent adequate penetration into cells and mitochondria. The penetration of glutamine into cells of kidney slices as well as in vivo will be investigated. In vivo studies will involve micropuncture experiments of tubular concentration of ammonia, glutamine and glutamate and measurements of A-V differences of glutamine and glutamate. Studies as to the possible role of mitochondria will continue. The relationship between the ability to excrete ammonia and mitochondria amount, glutaminase concentration and penetrability will be studied as a function of neonatal age.